John Henry Heuland, Fellow of the Geological Society, mineral collector and dealer of 25 King Street, St. James's, London, was born in 1778. He moved in 1839 to 31 Red Lion Square, London, where he remained until after 1850. Few details of his life are known, but his sale catalogues and his intense activity as a collector and dealer during the period when it was fashionable to form collections of minerals have secured him an important place in the historical archives of mineralogy.

In what year Heuland began to deal in minerals is uncertain, but it was before 1804 when we have a record of his buying in Lisbon. He subsequently travelled through France, Germany, Sweden, and Russia, collecting and buying minerals wherever possible. About the year 1806 he came into the possession of a fine collection of nfinerals which had been formed on the Continent between the years 1766 and 1806 by his uncle Mr. Jacob Forster.

The high-silica sericites generally contain a corresponding appreciable quantity of a divalent element, usually magnesium, and their chemical composition is interpreted as being intermediate between that of muscovite, KAl2 (AlSi3)O10(OH)2, and that of the equivalent high-silica mica leucophyllite, KAlMg (Si4)O10(OH)2. The series muscovite-leucophyllite includes the named micas phengite (high-silica sericite), mariposite, and alurgite. Selected analyses are plotted and fall on a corresponding straight compositional variation line from muscovite to leucophyllite. It is shown diagrammatically that the analysed sample of mariposite probably contained about 8 % of quartz. It is recommended that the only species names in this series to be retained are those of the end members muscovite and leucophyllite. Their indices of refraction and specific gravities are very similar.

In earlier analytical work on stony meteorites the analytical results were given in a number of different ways. Not only the principles, but also the methods used in the calculation of the mineral composition as derived from the chemical analyses, were of a different kind in almost every description of a stony meteorite. The same remarks of course apply to a certain extent also to the older descriptions of terrestrial rocks. The publications on the 'Quantitative [(American) system of[ classification of igneous rocks' (1) and some publications by Henry S. Washington, especially a paper on 'The statement of analytical results of rock analyses' (2), contributed greatly to the unifying of the methods of representing the results of chemical analyses of rocks and calculating their mineral composition. Later on other systems for the statement and calculation of rock analyses have been introduced by other petrologists, especially by Paul Niggli, and all this work, taken together, has greatly contributed to clarification of the different ways in which the results of rock analyses can be stated and made use of in the petrological study of rocks.

The study of minerals, using X-ray goniomcters in place of optical I instruments, has proceeded far in recent years. It is now a common practice among mineralogists and petrologists to confirm the nature of a given specimen by X-ray methods when there is some doubt about the identification by the older methods. The occurrence of pseudomorphs sometimes leads to misidentifications even in crystals of large size. If the material is cryptoerystalline it may be almost impossible to make an identification by optical methods. In such cases X-ray photographs may be of great assistance.